An optical disc device is a device for recording information or reading out recorded information on/from an optical disc using an optical head. In the optical disc device, a tilt angle between the optical disc and the optical head affects a performance of the device. If there is no tilt between the optical disc and the optical head, an information recording surface of the optical disc is irradiated with a better laser beam, and better recording/reproducing is performed. However, a tilt is generated between the optical disc and the optical head, an aberration occurs in a laser beam with which the information recording surface of the optical disc is irradiated, which deteriorates a recording/reproducing performance.
In general, the tilt includes a radial tilt and a tangential tilt. The radial tilt is a tilt between a vertical direction of a recording surface of an optical disc and a radial direction of an optical head. The tangential tilt is a tilt between the vertical direction of a recording surface of an optical disc and a direction vertical to the radial direction of an optical head (an optical head scanning direction).
FIG. 5 shows a relationship between an error rate and a radial tilt. FIG. 5 indicates clearly that the error rate increase when the radial tilt occurs. It expresses that a recording/reproducing performance is extremely deteriorated by occurrence of the radial tilt. Therefore, various methods are proposed for correcting the radial tilt to improve a reproduction signal.
For example, Patent Document 1 discloses a method in which an optical disc is irradiated with one main beam and two sub beams to detect a tilt, and the optical disc or an optical head is controlled so as to eliminate the tilt by using a tilt detecting signal.
Patent Document 2 discloses a method in which a quadrant photodetector is used and a radial tilt is detected based on a light volume balance of detectors in right and left sides with respect to an optical head operating direction (detectors divided in the radial direction), and a tangential tilt is detected based on a light volume balance of detectors in back and forth with respect to an optical head operating direction (detectors divided in a scanning direction) respectively so as to control those tilts.
Patent Document 3 discloses a method in which a rate of outputs from detectors in right and left sides with respect to an optical head operating direction (detectors divided in the radial direction) is varied along with the tilt. With this, an adverse effect due to the tilt can be eliminated by means of such as signal processing, without performing tilt control mechanically.
Patent Document 4 discloses a method in which an optical disc is irradiated with three or five beams, and a tilt is detected by using intensity of reflection of each beam. Then, depending on the tilt degree, one beam indicating the best reproduction signal quality is selected from among those three or five beams, and reproduced.
Patent Document 5 discloses a method in which a DPP and DPD signals are generated by using detection signals in a main spot and both side spots, or in the both side spots. A tilt angle of the optical disc is detected based on a subtracting value of those signals, and then a tilt angle is detected and controlled.
However, techniques in Patent Documents 1, 4 and 5 need a plurality of beams to be emitted, which leads to a complicated optical head. That is a great disadvantage. Further, the tilt detecting signal is obtained with no direct relationship to a recorded signal or a reproduction signal. Therefore, a recording or a reproducing characteristic is not necessarily improved optimally.
Further, the technique in Patent Document 4 uses one output from the three or five beams as a reproduction signal. In this case, each beam has to have a certain degree of light intensity from a viewpoint of obtaining a high SNR. Namely, a significantly high-powered laser diode is required to be used. That is not realistic.
A technique in Patent Document 2 has a comparably simple structure. However, the tilt detecting signal is not sufficiently accurate in such tilt detection. That is not practical.
A technique in Patent Document 3 also has a comparably simple structure. However, the largest problem exists in a way to set degrees to vary the rate of outputs from detectors divided in the radial direction. That is, in order to determine the degree to vary the output rate from the detectors, an optimal point needs to be obtained by measuring a jitter and an error rate with varying the output rate of the detectors. In this case, the error rate is an index indicating a difference between a bit string recorded actually and a bit string reproduced and decoded, and calculated by (an error bit)/(a total bit). The jitter and the error rate are values without polarity. Accordingly, when such a parameter is used for the adjustment, in a case with adjusting the tilt for example, an adjusting direction is unclear with respect to the tilt to be optimal state. Therefore, the tilt adjustment is performed with varying the tilt in positive and negative to fine the optimal point for a signal. Such a method is so-called a hill climbing. It takes a long time to learn for the optical disc device, and it is a serious deterioration in a comprehensive performance of the optical disc device. Further, Patent Document 3 also discloses a method in which information about the degree to vary the output rate is obtained in advance. However, the hill climbing is still necessary to obtain the information, and in addition, a tilt amount can be changed with the passage of time (a tilt is varied due to a temperature of an optical disc device or a humidity of an environment), so that sufficient accuracy cannot be expected with a correction amount previously stored.
Patent Document 1: Japanese Patent Application Laid-open No. 2003-157553
Patent Document 2: Japanese Patent Application Laid-open No. 2003-016677
Patent Document 3: Japanese Patent Application Laid-open No. 2001-256652
Patent Document 4: Japanese Patent Application Laid-open No. 09-054953
Patent Document 5: Japanese Patent Application Laid-open No. 2000-149298
Patent Document 6: Japanese Patent Application Laid-open No. 2004-213862